The present invention relates to a multi-channel echo cancelling method and a device thereof.
For a multi-channel echo cancelling method and a device thereof for cancelling an echo generated by propagation of reception signals in spatial acoustic paths, in a system having a plurality of reception signals and at least one transmission signal, a cascade connection type and a linear connection type are proposed in CS-84-178 (hereinafter referred to as Paper 1), pp.7-4, No.330, Vol.84 of the technical report of the Institute of Electronics and Communication Engineers.
The following problems, however, are pointed out in A5-3 (hereinafter referred to as Paper 2), pp.144-149, of proceedings of the 6-th digital signal processing symposium, that is, the cascade connection type and the linear connection type have following defects:
(1) its hardware size becomes large because its hardware size is in proportion to square of the number of channels; PA1 (2) convergence of adaptive filters becomes slow when reception signals have strong cross-correlation; and PA1 (3) coefficients of the adaptive filters may not converge to optimal values when reception signals have strong cross-correlation.
To solve these problems, a multi-channel echo canceller comprising an adaptive filter for each channel is proposed in A-202 (hereinafter referred to as Paper 3), pp.202, volume 1 of proceedings of the 1991 Institute of Electronics, Information and Communication Engineers spring conference.
Next, according to Paper 3, the case of applying this multi-channel echo canceller to a teleconference system, in which both of the reception signals and the transmission signals have two channels respectively, is explained.
FIG. 10 shows a block diagram of voice section of a two-channel teleconference system connecting two teleconference rooms 20 and 21.
An acoustic echo cancelling method in the first teleconference room 20 is explained here for an example.
Let us suppose that the second talker 9 is in the second teleconference room 21. The voice 10.sub.1 of the second talker 9 passes through the spatial acoustic path is recorded by the third microphone 11.sub.1 and is supplied to the second echo canceller 120.sub.2.
The voice recorded by the third microphone 11.sub.1 is sent to the first teleconference room 20 as the first reception signal 1.sub.1. Similarly to this, the voice 10.sub.2 of the second talker 9 passes through the spatial acoustic path and is recorded by the fourth microphone 112, and is supplied to the second echo canceller 120.sub.2. The voice recorded by the fourth microphone 11.sub.2 is sent to the first teleconference room 20 as the second reception signal 12.
In the first teleconference room 20, the first reception signal 1.sub.2 is played back by the first loudspeaker 2.sub.1, passes through the spatial acoustic path and comes to the first microphone 3.sub.1 and the first echo 7.sub.1 is generated at this time. The second reception signal 1.sub.2 is played back by the second loudspeaker 2.sub.2, passes through the spatial acoustic path and comes to the first microphone 3.sub.1 and the second echo 7.sub.2 is generated at this time. The echoes 7.sub.1, 7.sub.2 and the first transmission signal 8.sub.1 that is the voice of the first talker 6 come to the first microphone 3.sub.1 are added and become the first mixed signal 4.sub.1.
Similarly to this, the first reception signal 1.sub.1 is played back by the first loudspeaker 2.sub.1, passes through the spatial acoustic path and comes to the second microphone 3.sub.2 and the third echo 7.sub.3 is generated at this time. The second reception signal 1.sub.2 is played back by the second loudspeaker 2.sub.2, passes through the spatial acoustic path and comes to the second microphone 3.sub.2 and the fourth echo 7.sub.4 is generated at this time. The echoes 7.sub.3, 7.sub.4 and the second transmission signal 8.sub.2 that is the voice of the talker 6 come to the second microphone 3.sub.2 are added and become the second mixed signal 4.sub.2.
The first echo canceller 120.sub.1 is used to remove echoes 7.sub.1, 7.sub.2, 7.sub.3 and 7.sub.4 contained in the first and second mixed signals 4.sub.1 and 4.sub.2.
The propagation delay difference estimation circuit 110 receives the first reception signal 1.sub.1 and the second reception signal 1.sub.2, estimates a propagation delay difference that is a difference between propagation delays of voices 10.sub.1 and 10.sub.2 of the second talker 9 until reaching to the first teleconference room 20 from the second teleconference room 21, and supplies the result to the reception signal selection circuit 111.
The reception signal selection circuit 111, based on an estimation result of said propagation delay difference estimation circuit 110, detects a reception signal having shorter propagation delay from two reception signals 1.sub.1 and 1.sub.2. And it supplies the detection result as a designated reception signal selection information to the selector 105.
The selector 105 receives the first and the second reception signals 1.sub.1 and 1.sub.2, selects the reception signal 106 that is designated by said reception signal selection information from the two reception signals 1.sub.1 and 1.sub.2. And it supplies the result to the first adaptive filter 112.sub.1 and the second adaptive filter 112.sub.2.
The first adaptive filter 112.sub.2 receives the reception signal 106 selected by the selector 105, generates the echo replica 108.sub.1 corresponding to an echo contained in the first mixed signal 4.sub.1, and supplies it to the first subtracter 109.sub.1.
The first subtracter 109.sub.1 subtracts the echo replica 108.sub.1 that is an output of the first adaptive filter 112.sub.2 from the first mixed signal 4.sub.1 and lets the result be the first output signal 5.sub.1.
The first adaptive filter 112.sub.2 is controlled so as to make the first output signal 5.sub.1 be minimized.
Similarly to this, the second adaptive filter 112.sub.2 receives the reception signal 106 selected by the selector 105, generates the echo replica 108.sub.2 corresponding to an echo contained in the second mixed signal 4.sub.2, and supplies a result to the second subtracter 109.sub.2.
The second subtracter 109.sub.2 subtracts the echo replica 108.sub.2 that is an output of the second adaptive filter 112.sub.2 from the second mixed signal 4.sub.2 and lets the result be the second output signal 5.sub.2.
Here, the second adaptive filter 112.sub.2 is controlled so as to make the second output signal 5.sub.2 be minimized.
A conventional multi-channel echo canceller supposes that the propagation delay between the first reception signal 1.sub.1 and the second reception signal 1.sub.2 changes according to location of the second talker 9 in the second teleconference room 21.
However, there may be a microphone that scarcely generates propagation delay difference although generates power difference according to location of the second talker 9. When such a microphone is used, a conventional multi-channel echo canceler can not select an appropriate reception signal, so that it can not work properly. This has been a problem.